1. Field of the Invention
The present invention relates to an electrodeless discharge lamp and the manufacturing method of the lamp, and more particularly to a ceramic arc tube which contains metal halide with high vapor pressure, and a method for sealing the tube with a ceramic plate and molten glass.
2. Related Art of the Invention
As an electrodeless discharge lamp which inputs microwave to produce luminescent energy, a lamp with a quartz arc tube sealing sulfur and rare gas therein has become commercially practical (refer to the 7th International Symposium on the Science & Technology of Light Sources: B. P. Turner et al 1995, p.125). Furthermore, electrodeless ceramic discharge lamps in which an alkaline metal and inert gas are sealed with either monocrystalline alumina or polycrystalline alumina are being developed, as disclosed in Japanese Laid-Open Patent Application No. 54-119783.
However, the conventional microwave exciting high-pressure electrodeless discharge lamp with a quartz arc tube is poor in heat conductivity, so that the provision of a motor-driven support bar is necessary to heat uniformly the tube as shown in FIGS. 7a and 7b. An electrodeless lamps has a long life because of the absence of blacking which results from the evaporation of electrode materials. However, the life of the lamp depends on the durability of the motor which is needed to heat uniformly the tube.
On the other hand, since an alkaline metal in the electrodeless discharge tube which is sealed with either monocrystalline alumina or polycrystalline alumina is not in a halogenated state, it is believed that a tremendous power must be supplied to evaporate the alkaline metal and obtain an effective emission spectrum For this reason, an electrodeless discharge lamp with a ceramic arc tube sealing halide having high vapor pressure as luminescent material is not yet in the actual use.
In a sodium lamp which is the only ceramic discharge lamp that has become commercially practical, a cermet which is placed in the electrode sealing unit is induction-heated to melt the molten glass for the sealing. However, the induction-heating cannot be applied to the microwave electrodeless discharge lamp because of the absence of electrodes. A high-pressure sodium lamp which uses aniobium fine tube as the sealing unit has been in practical use. However, if the arc tube contains a metal inside the cavity which supplies energy, the metallic part in the cermet or niobium is locally heated, and as a result, the arc tube is easily destroyed.